The soldier boy in the poem was describing to the narrator what had happened the previous night. There was an explosion caused by the opponent army. And that was when the boy received the injury and pain in his chest and many of his fellow soldiers died.

Now coming to the expression ‘the night exploded’, night typically symbolizes darkness and silence. But the explosion caused sudden flashes and loud horrific sounds, thus breaking the darkness and silence of that night. That is somewhat like the night itself exploded. So, the poet has used it that way.

when the soldier boy and his fellow comrades had climbed the hill .Perhaps the hill was a mine field so the bombs exploded…Due to the loud explosion sound it felt as the night exploded and many of the other soldiers died in the explosion and the soldier boy experienced a wound which he calls a small pain in his chest.

The night exploded. Bombs burst with a big explosive sound all night. That was how the night exploded on the top of the hill. The young boy was hit either by bullet or splinter of the bomb. His chest received the wound due to such explosion in the night

The term night exploded means violent explosion during the night either by the land mines or by by firing guns at the same time as the soldier reach the top of the mountain. It was a sudden attack resulting in massive destruction and loss of life.